From time to time I need to talk about mathematics with my advisor remotely. I would like to approximate writing on a blackboard together as closely as is reasonable. What are some technological options I have for doing this?

Once I've heard a talk about this project: Media Spaces and Video in the Home at the Simon Fraser University, Canada. It featured a screen on which both parties were able to write text to be visible by both parties. Some screenshots may be seen on slides here. Maybe something like that could be applicable in your case?
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Alexander KonovalovSep 2 '13 at 19:52